The ability to silence genes via RNA interference (RNAi) was reported by Mello and Fire in 1998. See Fire et al., Nature (1998) 391:806-811. Since then, scientists have rushed to take advantage of the enormous therapeutic potential driven by targeted gene knockdown. This is evidenced by the fact that the first report of small interfering RNA (siRNA) mediated RNAi in human beings was reported only twelve years after the phenomenon was described in Caenorhabditis elegans. See Davis et al., Nature (2010) 464:1067-1070. It is well understood that development of genetic drugs is slowed by the inability to deliver nucleic acids effectively in vivo. When unprotected, genetic material injected into the bloodstream can be degraded by DNAases and RNAases, or, if not degraded, the genetic material can stimulate an immune response. See, e.g., Whitehead et al., Nature Reviews Drug Discovery (2009) 8:129-138; Robbins et al., Oligonucleotides (2009) 19:89-102. Intact siRNA must then enter the cytosol, where the antisense strand is incorporated into the RNA-induced silencing complex (RISC) (Whitehead supra). The RISC associates with and degrades complementary mRNA sequences, thereby preventing translation of the target mRNA into protein, i.e., “silencing” the gene.
To overcome difficulties in delivery, nucleotides have been complexed with a wide variety of delivery systems, including polymers, lipids, inorganic nanoparticles and viruses. See, e.g., Peer et al. Nature Nanotechnology, (2007) 2:751-760. However, despite promising data from ongoing clinical trials for the treatment of respiratory syncytial virus and liver cancers (see, e.g., Zamora et al., Am. J. Respir. Crit. Care Med. (2011) 183:531-538), the clinical use of siRNA continues to require development of safer and more effective delivery systems. Toward this end, numerous lipid-like molecules have been developed including poly β-amino esters and amino alcohol lipids. See, e.g., PCT Application Publication Nos. WO 2002/031025; WO 2004/106411; WO 2008/011561; WO 2007/143659; WO 2006/138380; and WO 2010/053572. Amino acid, peptide, polypeptide-lipids (APPL) have also been studied for a variety of applications, including use as therapeutics, biosurfactants, and nucleotide delivery systems. See, e.g., Giuliani et al., Cellular and Molecular Life Sciences (2011) 68:2255-2266; Ikeda et al., Current Medicinal Chemistry (2007) 14: 111263-1275; Sen, Advances in Experimental Medicine and Biology (2010) 672:316-323; and Damen et al., Journal of Controlled Release (2010) 145:33-39. However, there continues to remain a need to investigate and develop new APPL systems with improved properties, such as new and improved APPL nucleotide delivery systems.